1. Field of the Invention
This invention relates to a liquid crystal display device and its manufacturing method, especially suitable for application to a liquid crystal display device including a driving-purpose thin film transistor (TFT).
2. Description of the Related Art
Recently, liquid crystal display devices are used more often as light bulbs of projectors. Along with this tendency, liquid crystal display devices must have higher optical transmittance and higher definition. Liquid crystal. display devices using conventional techniques are explained below.
As shown in FIG. 1, a thin-film semiconductor layer 102 made of polycrystalline Si and having a predetermined configuration is provided on a quartz glass substrate 101 in a shading region, and a gate dielectric film 103 is provided on the thin-film semiconductor layer 102. On the gate dielectric film 103, a gate line 104 is provided. Although not shown, a source region and a drain region are formed in the thin-film semiconductor layer 102 in self-alignment with the gate line 104. The gate line 104 forming a gate electrode, and the source region and drain regions make up a polycrystalline Si TFT for driving pixel electrodes. On the gate dielectric film 103 in a predetermined location above the drain region, a holding capacitor line 105 is provided. The structure interposing the gate dielectric film 103 between the holding capacitor line 105 and the drain region constitute a holding capacitance element.
An inter-layer insulating film 106 is formed to cover the gate line 104 and the holding capacitor line 105. In predetermined locations of the inter-layer insulating film 106 and the gate dielectric film 103, contact holes 107, 108 are made. On the inter-layer insulating film 106, a lead-out electrode 109 is formed in connection with the drain region of the polycrystalline Si TFT through the contact hole 107, and a signal line 110 is formed in connection with the source region of the polycrystalline Si TFT through the contact hole 108. An inter-layer insulating film 111 is formed so as to cover the lead-out electrode 109 and signal line 110. The inter-layer insulating film 111 has formed a contact hole 112 in a predetermined position above the lead-out electrode 109. On the inter-layer insulating film 111, an upper-layer shading film 113 is formed in connection with the lead-out electrode 109 through the contact hole 112. The upper-layer shading film 113, lead-out electrode 109 and signal line 110 stacked together shade all region excluding pixel opening regions from incident light from above. Another inter-layer insulating film 114 is formed to cover the upper-layer shading film 113. The inter-layer insulating film 114 has formed a contact hole 115 in a predetermined position above the upper-layer shading film 113. On the inter-layer insulating film 114, a transparent pixel electrode 116 is provided in contact with the upper-layer shading film 113 through the contact hole 115. The pixel electrode 116 is covered by an orientation film 117 stacked thereon.
On the orientation film 117, a liquid crystal layer 118 is provided, which is covered by an orientation film 119 and an opposed common electrode 120. On the opposed common electrode 120, a transparent opposed electrode substrate 121 is provided.
In the liquid crystal display device having the above-explained configuration, a voltage applied to the transparent pixel electrode 116 connected to the thin-film semiconductor layer 102 forming TFT changes orientation of liquid crystal molecules in the liquid crystal layer 118 to control the display.
The signal line 110, gate line 104, holding capacitor line 105, thin-film transistor, and so on, are located within a inter-pixel shading region provided in the TFT substrate or opposed substrate. An example of such arrangement is shown in FIG. 9. FIG. 9 is an example of plan-view layout of the case where the signal lines 110 of the TFT substrate and the upper-layer shading film 113 form a shading region in a complementary fashion.
As shown in FIG. 9, in the conventional liquid crystal device, the gate line 104 and the holding capacitor line 105 extend in parallel, and the signal lines 110 extend to intersect with these gate line 104 and holding capacitor line 105. The lead-out electrodes 109 extend over the gate line 104 and the holding capacitor line 105 so as to bridge them in locations not overlapping the signal lines 110. Each upper-layer shading film 113 has a geometry bridging two adjacent signal lines 110 and covering parts of the holding capacitor line 105, the gate line 104 and lead-out electrode 109 located between these two signal lines 110. Each contact hole 108 is formed in a location of the signal line 110 overlapping an end portion of the thin-film semiconductor layer 102. The thin-film semiconductor layer 102 underlies the holding capacitor line 105 and the signal line layer 110. The holding capacitor line 105 has offset portions for avoiding the contact holes 107. Through each contact hole 107 in the offset portion, the thin-film semiconductor layer 102 and the lead-out electrode 119 are connected together. In the region where the lead-out electrode 109 and the upper-layer shading film 113 overlap, the contact hole 102 is formed to connect them. Further, in the region where the upper-layer shading film 113 and the holding capacitor line 105 overlap, the contact hole 15 is formed to connect them.
In the conventional liquid crystal device having the above-explained configuration, in order to realize a high optical transmittance and a high definition, it is necessary to reduce the inter-pixel shading regions of the liquid crystal display device.
However, according to the knowledge of the Inventor, signal lines 110, thin-film transistors, gate lines 104 and holding capacitor lines 105 occupy their respective areas in the conventional liquid crystal display device, and this is the bar against improvement of the pixel opening ratio.
It is therefore an object of the invention to provide a liquid crystal display device and its manufacturing method which can reduce the inter-pixel shading area and thereby realize a high optical transmittance and a high definition.
According to the first aspect of the invention, there is provided a liquid crystal display device having a thin-film transistor formed on a substrate for driving an pixel electrode, comprising:
a holding capacitor line underlying a thin-film semiconductor layer which forms the thin-film transistor; and
a first holding capacitor dielectric film formed between the holding capacitor line and the thin-film semiconductor layer,
a first holding capacitor element being made up of the thin-film semiconductor layer and the holding capacitor line interposing the first holding capacitor dielectric film therebetween.
In the first aspect of the invention, the a first insulating film is typically interposed between the thin-film semiconductor layer and the holding capacitor line. The first insulating film has a part which is thinner than the other part thereof and forms the first holding capacitor dielectric film. Further, in the first aspect of the invention, a recess is preferably formed in the first insulating film to make the portion thinner than the other part in the first insulating film, and the recess constitutes the first holding capacitor dielectric film.
In the first aspect of the invention, the holding capacitor line is typically set in a constant electric potential.
In the first aspect of the invention, the holding capacitor line is formed in a region overlapping and flatly cover a channel region in a thin-film transistor.
In the first aspect of the invention, the first holding capacitor dielectric film may be made of a silicon oxide film, silicon nitride film, or multi-layered film of a silicon oxide film and a silicon nitride film.
In the first aspect of the invention, the thin-film semiconductor layer forming the holding capacitor element is typically made of an extended portion of a diffusion layer in the thin-film transistor, and a conductive impurity is introduced into the extended portion to reduce its resistance.
In the first aspect of the invention, a holding capacitor electrode is typically provided on the thin-film semiconductor layer via a second holding capacitor dielectric film, and the holding capacitor electrode and the thin-film semiconductor layer underlying via the second holding capacitor dielectric film make up a second holding capacitor element.
Configuration of this holding capacitor electrode is the same as the configuration of the gate line in the thin-film transistor. More specifically, the gate line and the holding capacitor electrode have a structure stacking a tungsten silicide film on a polycrystalline Si film, for example. Still in the first aspect of the invention, the second holding capacitor dielectric film is made of a dielectric film which is used as a gate dielectric film in the thin-film transistor.
According to the second aspect of the invention, there is provided a manufacturing method of a liquid crystal display device having a driving thin-film transistor formed on a substrate, comprising the steps of:
forming a holding capacitor line on the substrate;
forming a first holding capacitor dielectric film on the holding capacitor line; and
forming a thin-film semiconductor layer constituting the thin film transistor on the first holding capacitor dielectric film so that the holding capacitor line and the thin-film semiconductor layer interposing the first holding capacitor dielectric film therebetween make up a first holding capacitor element.
Typically, in the second aspect of the invention, a first insulating film is formed on the holding capacitor line, and a portion thinner than the other part of the first insulating film is formed in the first insulating film so that the thinner portion of the first insulating film makes up the first holding capacitor dielectric film. Further, in the second aspect of the invention, by typically making the first insulating film on the holding capacitor line and making a recess in the first insulting film, the first holding capacitor dielectric film made of the recess in the first insulating film is formed.
In the second aspect of the invention, the holding capacitor line is typically made in a process different from the process for making a gate line forming the thin-film transistor.
In the second aspect of the invention, by preferably making a second holding capacitor dielectric film on the thin-film semiconductor layer making the holding capacitor electrode on a second insulating film, a second holding capacitor element made up of the holding capacitor electrode and the thin-film semiconductor layer is formed via the second holding capacitor dielectric film. At that time, for the purpose of reducing steps of the manufacturing process, the holding capacitor electrode is made simultaneously with the gate line of the thin-film transistor.
In the second aspect of the invention, preferably by making a second insulating film on the holding capacitor line, making an opening in the second insulating film to expose the surface of the holding capacitor line, and making on oxide film on the exposed surface of the holding capacitor line, the first holding capacitor dielectric film made of the oxide film is formed. For making the oxide film, anodic oxidation or annealing is conducted to form the oxide film on the exposed surface of the holding capacitor line. Thickness of the oxide film is preferably not thinner than 5 nm and not thicker than 300 nm, and more preferably, not thinner than 10 nm and not thicker than 100 nm.
In the present invention, typically used as the thin-film semiconductor layer is a polycrystalline Si film. It is also possible to use a single-crystal Si film, amorphous Si film, or a compound semiconductor film of gallium arsenide (GaAs), for example.
In the present invention, in order to secure a sufficient holding capacity Cs of the first holding capacitor element, thickness of the holding capacitor dielectric film is preferably not thinner than 5 nm and not thicker then 300 nm, and more preferably, not thinner than 10 nm and not thicker than 100 nm.
In the present invention, the holding capacitor line is made of tungsten, molybdenum, tantalum, chrome, titanium, tungsten alloy, molybdenum alloy, tantalum alloy, chrome alloy, titanium alloy, tungsten silicide, molybdenum silicide, tantalum silicide, chrome silicide, titanium silicide, or silicon doped with an impurity.
According to the liquid crystal display device and its manufacturing method having the above configuration according to the invention, since the holding capacitor line is provided under the thin-film semiconductor layer forming the thin-film transistor so that the thin-film semiconductor layer and the holding capacitor line stacked via the holding capacitor dielectric film make up the holding capacitor element, it is possible to configure the thin-film transistor and the holding capacitor line not to occupy their own areas, respectively, and rather make them in the region flatly overlapping them.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.